Dichlorosilane can be prepared from a reaction of FeSi and HCl. However, such a method of preparation produces a low product yield and requires a great deal of distillation to separate the numerous by-products. It therefore has become common practice to conduct a disproportionation of trichlorosilane on suitable catalysts to dichlorosilane and separate any silicon tetrachloride. However, that reaction is slow to attain equilibrium. Most of the catalysts used are secondary and tertiary amines or quaternary ammonium salts (cf. DE-AS 21 62 537). High temperatures and high pressures are used to accelerate achieving equilibrium and to avoid excessively long residence times in the catalyst bed and in the reactor. The use of a high pressure condition presents a high degree of risk because dichlorosilane and any formed H.sub.3 SiCl or SiH.sub.4 are selfigniting. Any leakage can result in extremely violent reactions. In flow-through reactors the content of unreacted trichlorosilane is very high. The trichlorosilane has to be repeatedly redistilled with a high energy consumption before a complete transformation is finally achieved.
The problem therefore was to reduce the high cost of energy and especially to reduce the amount of distillation required for the separation of unreacted trichlorosilane.
Another problem was to rapidly establish the equilibrium of the disproportionation and to remove the dichlorosilane immediately after its formation. Still another problem was to reduce the cost of the apparatus.